The present invention relates to infrared (IR) and ultraviolet (UV) absorbing soda-lime-silica glass compositions for use in glazing. More particularly, the present invention relates to windows of a neutral tint made from such glasses primarily, but not exclusively, for vehicles such as automobiles.
Special glasses have been developed for use in vehicles which have low levels of direct solar heat transmission (DSHT) and ultraviolet transmission (UVT). These glasses aim to reduce the problems caused by excessive heating within the vehicle on sunny days, and to protect the interior furnishings of the car from the degradation caused by ultraviolet radiation. Glasses having good infrared absorption properties are usually produced by reducing iron present in the glass to the ferrous state or by adding copper. Such materials give glasses a blue color. The materials added to achieve good ultraviolet radiation absorption are Fe.sup.3+, Ce, Ti or V. The quantities of such materials which are added to provide the desired level of absorption tend to color the glass yellow. Accordingly, if both good UV and good IR absorption are required in the same glass, the color of such glass is, almost inevitably, either green or blue. When the color of the glasses is defined by the CIELAB system, such commercial glasses, in 4 mm thickness and having greater than 60% light transmission, are found to be either very green (-a*&gt;8) or very blue (-b*&gt;7), neither of which are currently desirable from an aesthetic viewpoint.
Attempts have been made to produce grey or bronze-colored vehicle glazing having good protection against both IR and UV radiation, but such glasses still tend to have a greenish yellow tinge.
We have identified a requirement for a range of glasses having a neutral tint and a visible light transmittance (Illuminant A) of at least 70 percent such that, in the CIELAB system, the glasses have color co-ordinates lying in the ranges a* from -7 to +1, b* from -5 to +7.5. The term "neutral tint" is hereinafter used to describe glasses having such color co-ordinates.
We have further identified a requirement for glasses having a neutral tint which have visible light transmissions of at least 70 percent (at a thickness of 4 mm), but which also have a direct solar heat transmission which is at least twelve percentage points (preferably fifteen percentage points and most preferably twenty percentage points) less than the visible light transmission. Basically, glasses are known which do have a low direct solar heat transmission but nearly all of these have a low visible light transmission which tend to make such glasses of limited use in vehicles. Glasses satisfying the above-identified requirements should, we anticipated, be of more general use in vehicles due to the higher light transmission but the lower direct solar heat transmission should keep the interior of the car cool despite the higher light transmission.
Furthermore, we believed that it would be desirable if the glasses had an ultraviolet transmission less than 55% and ideally less than 50% because we felt that such a low transmission would minimize the adverse effects of ultraviolet radiation on plastics material and fabrics, particularly in automotive vehicles.
The field of tinted glasses is one in which relatively small changes can produce major changes in tint. Wide ranges disclosed in prior patents can encompass many possibilities, and it is only the teaching of the specific examples that can be relied on as identifying how particular tints associates with particular ranges of absorption of infrared and ultraviolet radiation can be obtained.
Our invention is based on the surprising discovery that the incorporation of relatively small amounts of certain coloring agents compensates for the green color arising from the presence of infrared and ultraviolet radiation absorbing components.